1. Field of the Invention
The present invention generally relates to wireless communications systems, and more particularly to methods for controlling data rate in such systems.
2. Description of Related Art
In a CDMA system, since voice communications cannot tolerate extensive delay, priority is typically given to the transmission of voice traffic over the transmission of data traffic. The amount of voice activity at any given instance is unpredictable and, therefore, in a CDMA system the transmission of data will be adjusted to prevent the reverse link capacity from being exceeded. In addition, since a mobile station may be in soft handoff between multiple cells, the data transmission rate assigned may be based on the reverse link capacity of each of the base stations involved in the soft handoff.
It is anticipated that the demand for higher transmission rates for data will increase for both the forward link and the reverse link, as mobile users are expected to receive and generate increased amounts of data. The forward link signal is the RF signal transmitted from a base station to one or more mobile stations, and a reverse link signal is an RF signal transmitted from a mobile station to a base station.
In anticipation of the need for a system to transmit data at higher rates, a high data rate (HDR) system has evolved. An HDR system, such as a system employing CDMA cdma2000 1xEV-DO technology, relies upon a dedicated 1.25 MHz channel to deliver data at speeds up to 2.4 Mbps. An HDR system shares the same spectrum with an IS-95 or IS-2000 system by using the aforementioned separate 1.25 MHz frequency dedicated to HDR. The HDR forward link uses a single shared channel which always transmits at full power (except for the power control bits). Users are time-multiplexed on the forward channel (instead of code division access) so as to achieve a higher data throughput for a single user. There is no softer/soft handoff on the forward link, since the mobile station only listens to one forward link at any given time. The transmission data rate on the forward link, as well as which sector (the sector is the part of a base station that provides one CDMA channel) in the next desired transmitting forward link is typically determined by the mobile station, with the mobile station indicating the desired or requested data rate in serving sector via a data rate control (DRC) channel on the reverse link.
The base station controls and selects which user's data is to be transmitted in a next forward link slot by using a scheduling algorithm. Each slot is 1.667 ms and each frame is 26.67 ms, as is known, and there are 16 slots per frame. The forward link consists of four types of channels: pilot, Medium Access Control (MAC), control and traffic channels, with the MAC supporting reverse activity, DRC lock and reverse power control channels, as is known. The reverse link has access and traffic channels, with the traffic channel being further divided into a pilot, MAC, ACK and data channel, and with the MAC supporting the DRC channel and a reverse rate indicator (RRI) channel, as is known. When a mobile station is assigned to a reverse traffic channel, the mobile always transmits the pilot, the DRC and the-ACK channels.
Accordingly, in an HDR system, data rate control is necessary for efficiency. In an HDR system, the mobile station typically requests a forward link data rate based on an estimation of a forward traffic channel condition. For example, in decoding a packet, a mobile station may measure a carrier-to-interference or signal-to-noise ratio (SNR) of the pilot signal received from the base station over the forward channel. The mobile station makes this SNR measurement and then adds a margin (a predetermined value) to the measured SNR in order to determine a predicted or estimated SNR, as is known. The predicted SNR is used in order to determine a rate control message that is to be sent to the base station. This rate control message, also known as a rate request message, is transmitted to the base station over the DRC channel on the reverse link. When the base station receives the rate request message, the base station adjusts the rate of the transmitted signals accordingly.
The margin, referred to as a DRC margin, is included to account for transmission delay. For example, there is a delay between the mobile station requesting a forward link data rate, based on estimation of a forward channel condition, and the base station sending the data packet at the requested data rate. Typically the delay is on the order of two slots. Accordingly, the DRC margin is typically set to some value in an effort to overcome the channel estimate errors due to this transmission delay.
To achieve a consistent quality of service (QoS), the value of the DRC margin should account for changing conditions, such as a different fading environment, or a different speed of the mobile station. However, the current existing algorithms set DRC margin at a fixed or predetermined value. Setting the DRC margin at a fixed value prevents efficient tracking of a changing environment which could waste system resources when the mobile station is at low speed and could degrade QoS, such as high encoded packet error rate (EPER) for example, when the mobile is at high speed.
As discussed above, the estimated or predicted SNR is used to determine the rate request message which is sent to the base station over the DRC channel on the reverse link. U.S. Pat. No. 6,416,971 to Wu et al. describes a system to predict signal-to-noise ratio. In FIG. 2 of Wu '971, a prediction selector 82 selects one of a plurality of outputs from signal-to-interference and noise ratio (SINR) predictors 76, 78 and 80. Particularly, prediction selector 82 selects a signal having a smallest standard deviation of prediction error value, which is most representative of the current fading signal environment. The selected SINR prediction is then sent to a rate request generation circuit 44 to determine a desired rate control message.
However, Wu et al. bases each of the individual SINR predictions on a fixed margin “D” which is a predetermined delay factor based on a given packet length. Accordingly, Wu et al. determines predicted SINR based on an open loop solution, using a fixed “guess” for the DRC margin. These fixed DRC margins are not accurate and do not account for transmission delay due to changing environmental conditions or speed of the mobile station, for example.
Additionally, setting a fixed DRC margin may lead to the following possible effects: (a) if there is too much margin used in determining estimated SNR, and estimated SNR is based on the measured SNR minus the DRC margin, then the requested data rate may be too low, because the target packet error rate would be zero percent. Thus system performance would be inefficient (e.g., every received packet would be good, indicating that the transmission rate is too low). If not enough margin is used, target packet error rate could exceed acceptable limits, (e.g., >>1%), thus there would be too much error. The requested data rate would be too high, wasting resources and leading to less efficiency due to the number of errors requiring retransmissions. Thus, using a fixed DRC margin, based on an open loop “guess”, is inaccurate and inappropriate for HDR systems.